Monthly Archives: May 2015

I have reached page 682 of the Odem book which is where I am going to stop. Now I am going finish typing up my notes. Next I will use the attached CD to quiz myself to figure out what areas I need to brush up on in the coming weeks.

CHAP19 Subnet Design p533
– count the bits know the powers of 2
– 2^10 is 1024 and that is easy to remember

CHAP20 VLSM p561
– Old routing protocol doesn’t support vlsm (RIP)
– no additional config to get this to work
– be able to find overlap of networks to troubleshoot

CHAP21 Route Summarization p577
– strategy used for performance to lower the size of routing tables
– subnet design should have summarization in mind
Steps to finding the best summary route
1. list all decimal subnets in order
2. note low and high points
3. pick the shortest prefix length mask and subnet -1
4. calculate new potential network mask summary

I took a couple 10 question tests from the CD. The idea was hit some chapters that I struggled with which were, WANs, ACLs and NAT. I got 6 out of 10 questions right which isn’t all the great.

Next I took a test of the first 5 chapters of the book. I scored 8 out of 10 right which is passing for the book test. The only concept I wasn’t sure on was crossover cable pin numbers and when to use a straight through and crossover cable. I knew like devices use crossover cables but that alone didn’t help me get the two questions right. I may memorize this table for the test.

I’m starting to see the fruits of an aggressive study plan. Here we are, May 23rd, roughly two weeks until test time and I am nearly on track.

Part I: Networking Fundamentals
Part II: Ethernet LANs and Switches
Part III: Version 4 Addressing and Subnetting (Be done by May 11th and practice subnetting)Part IV: Implementing IP Version 4 (Be done by May 18th and practice show commands)
Part V: Advanced IPv4 Addressing Concepts
Part VI: IPv4 Services (Be done by May 26th, decide if I want to skip IPv6, Review OSPF and practice more advanced subnetting)
Part VII: IP Version 6
Part VIII: Final Review (Be here by June 1st and have taken a practice exam to decide what areas to review)

I got off to a rocky start with an older 2008 version of the book. Fortunately my study buddy had purchased the correct book instead of borrowing an old one. I had gotten two chapters into the old book and before I started to really get into the newer edition that took a week to recieve. I decided to take a practice test early on. The test is very configurable. I chose study mode for 45 questions and limited myself to 90 minutes with a small chunk of whiteboard. I also decided to exclude any IPv6 questions from this first stab.

After two chapters and a couple videos on subnetting I was able to get a 600 which is 200 points away from passing. This was on the practice test that came on the CD in the book. The higher layer concepts I did quite well on where as the lower layer concepts such as Routing, WANs, ACLs and any kind of IOS commands and configuration questions I did very poorly on. Subnetting seems to get a lot of attention either directly, or indirectly and I was sitting at about 50% or less on that.

What is subnetting?

Don’t listen to me, I’m not an expert, but I don’t think there are many good explanations of this out there. A lot of people go way deep and off on tangents to frequently. Here is my overview of what I understand are important subnetting concepts for ICND1.

IP Address = 32 bits = 4 Octects = 4 bytes

Each byte can store 256 possible combinations of 1s and 0s. So lets represent 10.0.0.1 in binary, 00001001.00000000.00000000.00000001

See, that is 32 bits in an IP address.

The second concept we need to understand is the netmask. Picture a mask you might put on your face. A very thick mask you won’t be able to see much. A thin mask you might be able to see a lot.

Take that concept and apply it to this very common netmask 255.255.255.0, or 11111111.11111111.11111111.00000000

Out of all the possible combinations that is a pretty thick mask so I can only see a small number of hosts with that mask. If you combine the IP & netmask, you will be able to see IP address from 10.0.0.0- 10.0.0.255 or 256 possible hosts.

And there you have it, networking. Wait, what was I talking about? Ah yes, SUBnetting.

Subnetting takes those 256 possible hosts and divides them into smaller networks. If I needed several separate networks and only 18 hosts per network I could split that 10.0.0.0/24 network into smaller chunks. If I want to see fewer hosts in my network I need a thicker, or higher number mask.

Pulling up the /24 mask again, 11111111.11111111.11111111.00000000 you will see it is /24 because there are 24 1s or network bits and 8 0s or host bits.

In our problem, we need at least 18 IP address options for hosts. For this we will use 0s. How many 0s will we need? Less than 8 for sure because that gave me 256 options. But how many less?

The powers of 2 come in handy for any binary math. There are 2 possible values for each bit, 0 or 1. With 2 bits there are 4 possible values, 00, 11, 10, 01. That isn’t going to get me to at least 18 hosts. This could take a while and for the ICND1 test you need to subnet in 15 seconds. Yikes!

In comes the cheat sheet.

Memorize this formula to go with the table: Possible hosts on a network = 2^h – 2

Each network supports 2^h ip addresses, however 1 ip address is used for the network id and another is used for the broadcast address, hence the minus 2 part.

I don’t suggest just memorizing the table. I would suggest understanding how to generate the table. Start from the top right and do your powers of 2 up to 128. 2^0 = 1, 2^1 =2 2^2=4 … 2^7=128

Next is the second row, the decimal mask. Take 256 – the h row to get the decimal mask row.

Next is the last 2 octets of cidr notation. This is simply a count of 1s in the binary representation of the mask. Remember 1s are the network bits and 0s are the host bits.

Once we have this table we can solve our problem, subnet 10.0.0.0/24(think 10.0.0.0-255/24) in a way that supports at least 5 networks and at least 18 hosts in each network.

Start this question with the important number h, or 18.

Go to the table and find the h value that supports at least 18 hosts, which is 32.

Go down to the decimal notation .224 and we know that we can support at least 18 hosts with a decimal mask of 255.255.225.224.

Next we can list the network IDs that this mask could possibly create
10.0.0.0/27
10.0.0.32/27
10.0.0.64/27
10.0.0.96/27
…
10.0.0.224/27

To figure this out mathematically take 2^n where n = the number of network bits. There are 3 network bits or 1s in the octect we subnetted. We can make 8 networks which is greater than 5 required by the problem. BOOM CAKE!

For the remainder of this post I will be simply typing up my notes from the Wendell Odom Cisco Press Book and some other notes I took watching YouTube videos from a variety of authors which I will link to.

The more I learn about networks, the less I tend to blame the network.

It was almost 20 years ago that I set a static IP address on my sisters computer and connected a cross over cable to my computer so we could play a game called Quake. She wasn’t that interested so I ran back and forth between the rooms and played by myself. This loneliness was resolved a few years later with a device that looked something like this

Point is, I’ve been doing this for a long time and I still don’t know jack. I don’t like to fail tests, so signing up for a test is going to help me learn. I would like to become a more well rounded datacenter administrator.

ICND1 100-101 is the first half of a valuable certification CCNA. I now have the book in hand and about 5 weeks to prepare. Normally, I would allow myself about 3 months with a book this size but opportunity has struck and I need to accelerate my pace.

These do not line up that nicely to the book topics. But I am going to attempt to cruise through the book which I have given myself some milestones below.

Part I: Networking Fundamentals
Part II: Ethernet LANs and Switches
Part III: Version 4 Addressing and Subnetting (Be done by May 11th and practice subnetting)
Part IV: Implementing IP Version 4 (Be done by May 18th and practice show commands)
Part V: Advanced IPv4 Addressing Concepts
Part VI: IPv4 Services (Be done by May 26th, decide if I want to skip IPv6, Review OSPF and practice more advanced subnetting)
Part VII: IP Version 6
Part VIII: Final Review (Be here by June 1st and have taken a practice exam to decide what areas to review)

The schedule is set, plans are in place, now it is time for me to do some reading.